1. Field of the Invention
The present invention is related to high efficiency non-polar and semi-polar light emitting devices such as light emitting diodes (LEDs) and laser diodes (LDs).
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification. In addition, a list of a number of different publications can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
Currently, most commercial gallium nitride (GaN) based light emitting devices are constructed on c-axis oriented crystals. As a result, strong piezo-electric fields control light emission efficiency. In the structure of conventional LEDs and LDs, the thickness of the quantum well (QW) is only around 2-5 nm. When the thickness of the QW layer becomes larger than 5 nm, the conduction energy band and valence energy band of the QW layer is bent due to a strong piezoelectric field, and then the radiative recombination efficiency becomes small, and then the emitting efficiency of the LED and LD becomes smaller.
However, in the case of non-polar or semi-polar GaN, the piezoelectric polarization is minimized. So, the energy band bending in the QW is small. It is expected that the thickness of the QW can be increased to improve the emitting efficiency of the LED and LD. Recently, the inventors of the present invention found that the thickness of the QW should be increased up to 10 nm in the case of non-polar and semi-polar LED and LD to increase the emitting efficiency.
In the case of conventional LEDs, in order to increase the light output power from the front side of the LED, the emitted light is reflected by a mirror placed on the backside of the substrate or is reflected by a mirror coating on the lead frame, even if there are no mirrors on the backside of the sapphire substrate, and if the bonding material is transparent on the emission wavelength. However, this reflected light is re-absorbed by the emitting layer (active layer), because the photon energy is almost same as the band-gap energy of the light emitting species, such as AlInGaN multiple quantum wells (MQWs). The efficiency or output power of the LEDs is decreased due to this re-absorption of the LED light by the emitting layer. See, for example, FIGS. 1, 2 and 3, which are described in more detail below. See also J. J. Appl. Phys. 34, L797-99 (1995) and J. J. Appl. Phys. 43, L180-82 (2004).
What is needed in the art are LED and LD structures that generate light more efficiently in the blue, green, amber, and red regions and that more effectively extract light. The present invention satisfies that need.